Two of the most unusual lunar impact craters on the Moon lie just east
of the Apollo 11 landing site (AS15-M-2045; Figure 1). Messier is an
unusual elliptical crater with a very distinctive "butterfly" ejecta
pattern. Nearby Messier A is actually a complex structure called a
"doublet" crater, or two small circular craters in close proximity to one
another.

In order to better understand the crater forming process, scientists
fired miniature asteroids into lunar-like targets. Real asteroids hit the
Moon at fantastically high speeds, greater than 16 km per second (or
35,000 miles per hour). The vertical gun facility at the
NASA Ames Research Center provided the best opportunity to simulate these
high-energy events. One type of experiment involved tilting the gun
at a very steep angle. As the angle of impact got steeper and
steeper, scientists saw no change from the typical circular crater.
Finally, when the angle was grazing (or less than 15&deg from the horizon)
elliptical, rather than circular, craters formed. Additional ray
patterns were seen to spread out like butterfly wings at these extreme
impact angles. The high-velocity gun experiments led scientists to
the hypothesis that Messier formed as a result of a low-angle impact.

The origin of Messier A is more enigmatic. It is possible that
Messier A formed in the same impact event as Messier, but questions
about the origins of the doublet crater feature exist. Suggestions
for the doublet include an impact near a pre-existing crater or the
near-simultaneous impact of two small pieces that broke off from the
same projectile that formed Messier. Messier and Messier A are
complex features, and geologic investigation of this region by human
explorers will clearly be required to determine how Messier A formed.

The Lunar Reconnaissance Orbiter
Camera, launching aboard the United States' Lunar Reconnaissance
Orbiter spacecraft in early 2009, will photograph craters like
these on the lunar surface at 0.5m/pixel. These new high-resolution images
will help us to not only improve our understanding of lunar craters,
but will also let lunar scientists plan the exploration traverses that will be
performed by the next generation of lunar explorers near interesting
features like Messier and Messier A.